The present invention is related to optical lenses and, in particular, an image pickup lens system used in a video camera.
Recently, reduction in size and weight of a video camera, i.e., its miniaturization, has become a significant and almost indispensable factor in bringing a successful product to market. It is apparent that the principal route to the miniaturization of the camera is to decrease the size of an image pickup lens system.
Namely, it is considered that largely two elements in the image pickup lens system determine the progress of camera miniaturization: the size of a solid-state image sensing device, i.e., a charged-coupled device (CCD), and the number of parts in a lens array. Various ways to reduce parts in the lens array, for example, are to include an inner focus lens, as disclosed in Japanese Patent Application Laid-Open No. 62-24213 (1987), or an aspherical lens, as disclosed in Japanese Patent Application Laid-Open No. 3-33710 (1991).
In the image pickup lens system of the video camera such as described in the above-mentioned patent applications, lenses that are components of the image pickup lens system are arranged in a straight line. That is, an optical axis extending from an object side to an image side forms a straight line. As a result, the miniaturization of a video camera with such a lens arrangement is quite limited, even if other steps for miniaturization have been taken into account. Clearly, the lenses arrayed along a straight line present an obstacle to the overall camera length reduction and impose restrictions on the camera design.
In addition or in place of a general lens, a zoom lens has been used frequently in the image pickup lens system of the video camera. Since the zoom lens is typically larger than the general lens, the problem of miniaturization is even more pronounced in this case.
To reduce the overall camera size with either the zoom lens or general lens, it is possible to arrange the lenses in a non-linear configuration, i.e., to fold the optical axis of the image pickup lens system so that the lenses are positioned at right angles to each other, for example. Namely, as disclosed in Japanese Patent Application Laid-Open No. 7-13073 (1995), a mirror is located between a wide-angle converter lens and a master lens (zoom lens) for folding the optical axis of the system. However, the attendant deterioration of the imaging performance has been difficult to solve in these devices due to mass production errors. Since high precision in assembling the lens system is difficult to achieve during mass production, this problem has been greatly exacerbated in image pickup lens systems with non-linear optical axes.
It is also desirable to provide a wide-angle lens, in combination with the zoom lens, in the video camera. The field of view of the wide-angle lens should be as wide as possible. The wide-angle lens, as disclosed, for example, in Japanese Patent Application Laid-Open No. 6-324265 (1994) is composed of four groups (positive, negative, positive and positive groups, counting from the object to the image side) and has a field angle of 60 degrees.
Therefore, it appears that a miniaturized zoom lens having a wide angle field of view can be obtained by merely combining the techniques disclosed in the above-mentioned two applications. Various problems, however, are encountered in such apparently simple combination.
In particular, in the image pickup lens system disclosed in the 7-13073 application, a mirror is positioned between the wide-angle converter lens and the master lens (zoom lens) to fold the optical axis of the image pickup lens system. To achieve the desired wide angle field of view, the master lenses would have to be enlarged causing a corresponding increase in the physical size and weight of the camera. Similarly, if a reflecting member such as a mirror is added to the zoom lens of the 6-324265 application to fold the optical axis, the lens must be also increased in size to provide the desired zoom function.
Thus, it has been very difficult to obtain miniaturization of a video camera which combines, for example, a zoom lens with a wide-angle lens.